The invention relates to a gas and/or vapor discharge lamp having a glass lamp vessel which is sealed in a vacuum-tight manner and which is filled with a metal vapor and a rare gas, and more particularly to such a lamp which is intended to be operated with a high-frequency supply voltage (hereinafter referred to as a "discharge lamp"). An electric discharge is produced in the lamp envelope, and a transparent conductive layer is placed around a wall surface to surround the discharge.
The expression "a lamp operated at high frequency" is to be understood herein to mean a lamp which is operated with a supply voltage at a frequency of more than approximately 20 kHz.
A discharge lamp of the aforementioned kind is, for example, a low-pressure mercury vapor discharge lamp having a lamp vessel provided with electrodes which are connected to an electronic circuit for high-frequency operation, a low- or high-pressure sodium vapor discharge lamp operated at high frequency, or a so-called electrodeless discharge lamp, in which a high-frequency electromagnetic field is induced in the lamp vessel by means of, for example, a core of magnetic material, such as ferrite.
A problem occurring during the operation of the aforementioned discharge lamps, especially with electrodeless gas discharge lamps, is that electromagnetic fields are produced outside the lamp envelope in the vicinity of the lamp, which cause high-frequency interference currents in the supply mains; international standards, such as VDE, CISPR and FCC standards, apply both to the strength of the electromagnetic fields outside the lamp and to the value of the interference currents. These standards indicate a limit for the maximum value of the interference.
From the published Japanese Patent Application No. 51-78660, an electrodeless low-pressure mercury vapor discharge lamp operated at high frequency is known which has a lamp vessel the inner wall surface of which is provided with an interference-suppressing transparent conductive layer, which is connected to a wire-shaped lead-through member in the wall of the lamp envelope, this lead-through member being connected to earth in the operative condition of the lamp. This Patent Application describes that the strength of the electromagnetic fields outside the lamp envelope is reduced. However, it has been found that in these lamps, in which the conductive layer is connected to earth, disturbing electric interference currents were obtained in the supply mains. This is disadvantageous because the aforementioned standards cannot then be satisfied.